NRAO Background Information

Deciphering GRB Physics with Radio Telescopes

For 30 years, Gamma Ray Bursts, now known to be the most energetic
explosions in the sky, have intrigued scientists and constituted one
of the greatest mysteries in astrophysics. Such basic details as their
exact locations in the sky and their distances from Earth remained
unknown or subject to intense debate until just last year.

With the discovery of "afterglows" at X-ray, visible, infrared and radio
wavelengths, scientists have been able to study the physics of these
explosive fireballs for the first time. Radio telescopes, the NSF's
VLA in particular, are vitally important in this quest for the
answers about Gamma Ray Bursts. Planned improvements to the
VLA will make it an even more valuable tool in this field.

Since their first identification in 1967 by satellites orbited to
monitor compliance with the atmospheric nuclear test ban, more than 2,000
Gamma Ray Bursts have been detected. The celestial positions of the
bursts have only been well-localized since early 1997, when the Italian-
Dutch satellite Beppo-SAX went into operation. Since Beppo-SAX began
providing improved information on burst positions, other instruments,
both orbiting and ground-based, have been able to study the afterglows.
So far, X-ray afterglows have been seen in about a dozen bursts,
visible-light afterglows in six and radio afterglows in three.

The search for radio emission from Gamma Ray Bursts has been an
ongoing, target-of-opportunity program at the VLA for more than four
years, led by NRAO scientist Dale Frail.

The detection of afterglows "opens up a new era in the studies of
Gamma Ray Bursts," Princeton University theorist Bohdan Paczynski
wrote in a recent scientific paper. Optical studies of GRB 970508
indicated a distance of at least seven billion light-years, the first
distance measured for a Gamma Ray Burst. VLA studies of the same burst
showed that the fireball was about a tenth of a light-year in diameter
a few days after the explosion and that it was expanding at very
nearly the speed of light. Optical studies of a December 1997 burst
(GRB 971214) indicated a distance for it of nearly 12 billion light-years.

With distances known, astronomers could calculate the amount of energy
released during the explosion. The answers were astounding. GRB 970508,
in a mere 15 seconds, released nearly ten times more energy than our
Sun will release in its entire, 10-billion-year lifetime. GRB 971214,
for one or two seconds, outshone the entire rest of the universe.
These energies ruled out many of the numerous theories for the origin
of Gamma Ray Bursts that had arisen over the previous three decades.

Many answers about the origins of Gamma Ray Bursts and the physics
of the fireballs will come from radio telescopes. The VLA, with its
combination of sensitivity and resolving power, "has a unique role
to play in deciphering GRB fireball physics," said Dale Frail of the
National Radio Astronomy Observatory (NRAO) in Socorro, NM.

First, radio astronomers can see the GRB fireball far longer than
it is visible at other wavelengths. A Gamma Ray Burst is visible in
the gamma rays for typically seconds or minutes, in X-rays for days,
and in visible light for weeks, based on the past year's
experience.

"With radio telescopes, we can see the fireballs for months, gaining
new information every day," said Greg Taylor, also of NRAO in Socorro.
"Also, at other wavelengths, they see the emission only as it is
rapidly getting weaker. At radio wavelengths, we can study the emission
as it rises in strength, peaks, then slowly decays."

In addition, radio observations can measure the size of the fireball.
"Only radio telescopes can measure the size, and we can do it in
three different ways," Frail said. These techniques involve studying
the scintillation, or "twinkling" of the radio emission; absorption
characteristics of the emission; and, for bright, energetic afterglows,
direct measurements of sizes can be made through the great resolving power
of continent-wide radio telescope arrays such as the NSF's Very Long
Baseline Array (VLBA). The VLBA, along with other radio telescopes in
Europe and elsewhere, can measure sizes with extreme precision.

Also using the VLBA and other such arrays, radio astronomers can place
lower limits on GRB distances by measuring parallax, the apparent change
in an object's position in the sky caused by the Earth's motion around the
Sun.

Sensitive radio observations can pinpoint the position of a faint GRB closely
enough for optical astronomers to find it when otherwise they might not.
"This has happened already," explained Taylor. "We saw GRB 980329 with
the VLA and provided the position to the optical and infrared astronomers,
who then reexamined their images and discovered an extremely faint and
fading optical GRB counterpart at that location."

"Over the course of months, we can watch the spectrum of the radio
emission change," said Frail. "The bottom line is that the long-lived
and multi-faceted nature of the studies that can be done at radio
wavelengths makes radio telescopes a critically important tool for
investigating the nature of GRB fireballs."

NRAO scientists and engineers have been working for several years on
plans for updating the equipment at the VLA and expanding the
instrument's capabilities. The
planned upgrade would allow astronomers
to see more GRB afterglows and follow them longer. "We know already
that GRB afterglows are faint radio emitters. We're only seeing the
brightest of them," Taylor said. "With the VLA's sensitivity improved
as we envision it, we will see all the GRB radio afterglows."